1. Field of the Invention
The present invention relates to a novel electromagnetic wave absorption material and associated devices. The invention relates to various articles particularly, such as, an electromagnetic wave absorption material for millimeter wave of 1 to 300 GHz band, a printed wiring board that uses such absorbing material, an electronic device, a casing for an electronic device, a module for optical transmission or reception, automated tollgate, and high frequency communication equipment.
2. Description of the Prior Art
In late years, the high speed processing in electronic devices is spreading at an accelerating pace together with rapidly raised operating frequency of ICs and LSIs in microprocessors. This raised frequency allows such devices to easily emit undesired noise.
Moreover, in the telecommunications field, the next-generation multimedia mobile communication (on 2 GHz), the wireless LAN (on 2 to 30 GHz), and the high-speed telecommunication network on the optical fiber are presently in use. Further, 5.8 GHz in ETC (an automated Electronic Toll Collection) system in ITS (Intelligent Transport System), 76 GHz in AHS (an advanced cruise-assist highway system) are also in use. Therefore, a rapid expansion of use-range toward higher frequencies is continuously anticipated.
Rise in frequency of a radio wave causes more eased emission in a form of noise. At the same time, narrowed noise margin in the recent electronic device because of lowered power consumption therein, together with poor quality in noise-ambient inside an apparatus because of miniaturization and dense mounting in the electronic device, brings a malfunctioning problem due to EMI (Electro-Magnetic Interference)
Under these circumstances, a measure such that a radio wave absorbing material is installed inside an electronic device is typically adopted to reduce EMI that will appear inside the device. One of radio wave absorbing materials so far used is a composite sheet of electrically insulating organic matters, such as rubber and resin, and magnetically lossy material, like soft magnetic metal oxides having spinel crystal structure and soft magnetic metals. These techniques were disclosed by Japanese Patent application laid-open No. 7-212079, 9-111421, 11-354973, 11-16727, etc.
However, the relative permeability of a soft magnetic metal oxide of spinel crystal structure sharply reduces in the GHz-band according to the Snoek's law of threshold. This means that the usable frequency range for such metal oxide as an electromagnetic wave absorption material is up to several-GHz. In a soft magnetic metal on the other hand, its marginal frequency usable as a electromagnetic wave absorption material can be expanded up to about 10 GHz by use of eddy current suppression effect and shape-dependent magnetic anisotropy effect both yielded from making the particle therein into a flat-shape having a thickness thinner than the skin depth. However, such magnetic material prevents realization of light-weight electromagnetic wave absorber because of its inherent weightiness.
Japanese Patent application laid-open No. 2001-358493 has disclosed an integrated electromagnetic wave absorption material composed of particulate magnetic metal and ceramic, and those articles or facilities, which use said integrated absorption material, such as a printed wiring board, an electronic device, a casing for electronic device, an optical transmission or reception module, an automated tollgate, a high frequency telecommunication equipment, etc.
However, said magnetic material is still unsatisfactory in its electromagnetic wave absorption performance. Thus, development of an electromagnetic wave absorption material having an excellent performance in the extended use up to millimeter wave region is continuously desired. Although the electromagnetic wave absorption material according to said Japanese Patent application laid-open No. 2001-358493 is satisfactory for sub-millimeter wave region up to 5.8 GHZ, the use in the millimeter wave region over such frequency does not satisfy with the performance of said material.